Treating pressure vessels

ABSTRACT

A pressure vessel has a cylindrical side wall and a closed end joined to the side wall at a knuckle. The fatigue resistance of the pressure vessel is improved by autofrettage which moves a region of peak stress from the internal surface of the vessel to a region within the knuckle. Preferred are pressurized gas vessels of 6000 or 7000 series aluminum alloys.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention concerns pressure vessels, for example, high pressure gascylinders.

2. Discussion of Prior Art

Such pressure vessels are currently manufactured in aluminium, steel andcomposite materials. These vessels need to have excellent fracture andfatigue properties. Repeated cycling of pressure inside the vesselcauses the vessel to flex, and flexing encourages propagation of anycracks that may appear at the metal surface. Fatigue crack initiationand growth in such vessels occurs at those points where pressure cyclingcauses maximum flexing (change in strain). This invention concernstreatment of pressure vessels to improve their resistance to fatigue andprevention of premature burst failure.

An established method for improving the fatigue resistance of tubes andcylinders is known as autofrettage. This involves applying a pressurewithin the bore of the cylinder or tube sufficient to plastically deformthe metal at the inner surface. The technique produces compressiveresidual stresses near the bore, and thus enhances the fatigueresistance of the tube or cylinder subjected to cyclic internal pressureloading. The technique has been applied to continuous lengths of thickwalled tubing for at least 70 years.

Autofrettage has also been applied to pressure vessels known as fullwrap cylinders, whereby generally a complete thin-walled metal e.g.aluminium inner liner is put into compression. This invention is notconcerned with full wrap cylinders of that kind.

U.S. Pat. No. 3,438,113 describes the application of autofrettage tometallic pressure vessels, with the object of increasing the permissibleinternal pressure loading of the vessel. The invention involvesperforming autofrettage with the vessel at elevated temperature.

SUMMARY OF THE INVENTION

Fatigue failure of pressure vessels such as high pressure gas cylindersparticularly those with flat bottoms normally occurs, not in thecylindrical wall, but at or adjacent the closed end of the vessel. Thisinvention arises from the idea that the autofrettage technique might beused to improve the fatigue performance of such closed-end vessels.

In one aspect the invention provides a method of treating a pressurevessel of aluminium or an Al alloy, having a cylindrical side wall and aclosed end and having, when at service pressure, at least one region ofpeak stress located at an internal or external surface of or adjacentthe said closed end,

which method comprises subjecting the inside of the vessel toautofrettage by applying a pressure sufficient to plastically deform thesaid at least one region, said plastic deformation being confined toless than 25% of the wall thickness,

whereby the treated pressure vessel has the property that, when atelevated pressure, each region of peak stress is located away from anyinternal or external surface at a distance less than 25% of the wallthickness from said internal or external surface.

A region of peak stress is defined as one where the local stressdecreases in all directions with increasing distance from the region.

An effect of this treatment is to reduce the absolute value of the peakstress (when the cylinder is under any pressure above atmospheric andless than the autofrettage pressure) in the region of stress raisers(discussed below), and to move the position of peak stress away from asurface of the vessel. Thus in another aspect the invention provides apressure vessel of aluminium or an Al alloy having an axis, acylindrical side wall and a closed end joined to the side wall at aknuckle, and having the property that, when at elevated pressure, aregion of peak stress is located, within the material of the vessel awayfrom any internal or external surface at a distance less than 25% of thewall thickness from said internal or external surface, in the knuckleand/or axially of the vessel in the closed end. Preferably the saidregion of peak stress is located within the material of the vessel atleast 0.5 mm away from any internal or external surface.

Surface flaws are tears, pits, creases and are typically up to 1-200 μmdeep. If regions of peak stress coincide with these surface flaws, theytend to propagate. Moving regions of peak stress at leas 0.55 mm intothe interior of the material of the vessel should reduce or avoid thisproblem.

Autofrettage is normally performed at ambient temperature. Attemperatures substantially above ambient, the creep properties ofaluminium become more pronounced, and this progressively reduces thebeneficial effects of autofrettage.

The vessel may be of any aluminium (including alloys where aluminium isthe major component) material that can be formed into an appropriateshape and provide sufficient properties such as mechanical strength,toughness and fatigue and corrosion resistance. Among aluminium alloys,those of the 2000, 5000, 6000 and 7000 Series have been used to makepressure vessels and are preferred for this invention. The vessel ispreferably formed by extrusion.

Although hot extrusion according to the invention is possible, cold orwarm extrusion is preferred as being a lower cost procedure. Cold orwarm extrusion may also give rise to an extrudate having a bettercombination of strength and toughness properties. The preferredtechnique is backward extrusion. This technique involves the use of arecess, generally cylindrical, with parallel side walls, and a ram toenter the recess, dimensioned to leave a gap between itself and the sidewalls equal to the desired thickness of theextrudate. An extrusionbillet is positioned in the recess. The ram is driven into the billetand effects extrusion of the desired hollow body in a backwardsdirection. The forward motion of the ram stops at a distance from thebottom of the recess equal to the desired thickness of the base of theextruded hollow body. Extrusion speed, the speed with which theextrudate exits from the recess, is not critical but is typically in therange 50-500 cm/min. Lubrication can substantially reduce the extrusionpressure required.

The initial extrudate is cup-shaped, with a base, parallel side wallsand an open top. The top is squared off and heated, typically inductionheated to 350-450° C., prior to the formation of a neck by swaging orspinning. The resulting hollow bodyis solution heat treated, quenched,generally into col water, and finally aged.

The requirements of backward extrusion place constraints on the shape ofthe closed end of the resulting vessel, particularly the base and aknuckle by which the base is joined to the cylindrical side wall. Otherproduction techniques may place other constraints on the geometry of thevessel.

The inventors have performed finite element analysis which shows tht themajor stress raisers in such hollow bodies are located in two places: onthe inside of the vessel at the knuckle where the base joins the sidewall; and on the outside of the vessel at the centre of the base. Therelative values of these stress raisers may depend on the cylinder walland base thicknesses, the dimensions particularly the diameter of thevessel, and the particular base geometry chosen, especially the internalbase radius of the knuckle. The method of the invention involvesapplying a pressure within the vessel sufficient to cause plasticdeformation of the metal at one or both of these regions. The appliedpressure must obviously not be so great as to burst the vessel, and ispreferably less than that required to cause plastic deformation of metalthroughout the thickness of the base or knuckle. The applied pressuremay be such as not significantly to plastically deform the side wall ofthe vessel. Alternatively, any plastic deformation of metal in the sidewall should be confined to a region at or adjacent the inner surfacethereof, e.g. less than 25% and preferably less than 10% of the wallthickness.

The effectiveness of autofrettage in improving fatigue performance doesdepend on the design of the closed end of the pressure vessel. Thus forexample pressure vessels with hemispherical closed ends do not haveregions of peak stress and do not show the advantages of autofrettagedescribed herein. More usually, the closed ends of pressure vessels willhave semi-ellipsoidal or torispherical dish shapes, and the fatigueresistance of these can generally be improved by autofrettage asdescribed herein. For further description of these shaped ends,reference is directed to an ASME boiler and Pressure Vessel PublicationCode 1, Section VIII, Divisions 1 and 2. The effect of end shape isfurther described in Example 7 below. As there explained, positiveadvantages do result from designing a pressure vessel with a closed endjoined to a cylindrical side wall by a knuckle whose fatigue propertiescan be improved by autofrettage.

Aluminium high pressure gas cylinders are usually designed so that thestress in the cylindrical side wall at service pressure does not exceedhalf the alloy yield stress, and that the cylinder burst pressure is atleast 2.25 times the operating pressure. In a 7000 Series alloy cylinderhaving for example a yield stress of 450 MPa, the design should be suchthat wall stresses do not exceed 225 MPa. Bearing in mind the requiredburst pressure, it is possible to calculate the degree ofover-pressurisation needed for the internal surface of the cylindricalside wall to start to yield. (Wall stresses at the service pressure arehigher at the internal surface unless an autofrettage effect isinvolved). Calculations for a 175 mm diameter 7000 series alloy cylinderhaving yield atrength of 450 MPa and a wall thickness of 7.9 mm showthat pressurisation to at least 85% and often more than 95% of the burstpressure is needed before the stresses in the cylinder side walls exceedthe yield stress. Thus treatment of these cylinders by autofrettage ispossible under conditions which do not cause plastic deformation in theside wall. Indeed such treatment is advantageous, for autofrettage atpressures close to the actual burst pressure may lead to problems inmanufacture owing to variability in material properties, which forexample may lead to unwanted permanent expansion ofthe cylinder (BS5045: Part 3: 1984, Section 20.4, Volumetric Expansion Test) andtherefore would not be recommended as a commercial practice.

The autofrettage pressure is likely to be from 75 to 95%, e.g. 75 to90%, of the burst pressure of the vessel. A finite element analysis ofthe effects of the over-pressurisation can be performed to show that theright sort of residual stresses are obtained.

Finite element analysis (FEA) is a useful and powerful technique fordetermining stresses and strains in structures or components too complexto analyse by strictly analytical methods. With this technique, thestructure or component is broken down into many small pieces (finitenumber of elements) of various types, sizes and shapes. The elements areassumed to have a simplified pattern of deformation (linear or quadraticetc.) and are connected at “nodes” normally located at corners or edgesof the elements. The elements are then assembled mathematically usingbasic rules of structural mechanics, i.e. equilibrium of forces andcontinuity of displacements, resulting in a large system of simultaneousequations. By solving this large simultaneous equation system with thehelp of a computer, the deformed shape of the structure or componentunder load may be obtained. Based on that, stresses and strains may becalculated (See “The Finite Element Method”, 3rd Edition, the thirdexpanded and revised section of “The finite element method inEngineering Science”, O. C. Zienkiewicz, McGraw Hill Book Company (UK)Ltd, 1977).

The results of such finite element analysis are shown in FIGS. 1 and 2of the accompanying drawings, each of which is a von Mises Stress Plotof the lower part of the cylindrical side wall, the knuckle and half thebase of an aluminium high pressure gas cylinder repressurised to 24.1MPa. These were generated using a commercially available ANSYS computerprogramme, versions 5.0 or 5.1.

These FIGS. 1 and 2 show part of a 175 mm diameter cylinder having aparticular base profile, a burst pressure of 49.7 to 51.8 MPa and anassumed working pressure of 24.13 MPa (i.e. 1.17 times the normal designservice pressure). The von Mises plot of the residual stress is a usefulguide to the stress distribution. In each figure, contour lines withinthe wall and base of the pressure vessel are lines of equal stressvalue, the values of which are indicated by the letters A to I.

Referring to FIG. 1, the highest von Mises stress components are shownat the inner surface of the internal knuckle radius (371 MPa) and at theexternal surface in the centre of the base (377 MPa).

FIG. 2 shows the position again at the assumed working pressure of 24.13MPa but after autofrettage at 44.82 MPa (i.e. 90% of the theoreticalburst pressure). The peak von Mises stress at the knuckle has beenreduced to 145 MPa and is positioned a few mm away from the internalsurface. The peak stress at the centre of the base has been reduced to avalue below 282 MPa and is now positioned several mm from the externalsurface. In both cases, the depth of the peak stress component is nowmuch greater than the depth of any likely surface flaw. These twoeffects, the reduction in peak stress and its location change shouldlead to significant increases in the number of loading cycles needed toinitiate fatigue crack from a surface flaw.

These computer predictions are borne out in practice, as demonstrated inthe examples below.

Any point in a gas cylinder is in a complex stress state, that is, eachpoint is stressed in more than one direction, such as stresses in thehoop direction, in the radial direction and in the longitudinaldirection.

Description of Stress at a Point and Principal Stresses:

In solid mechanics, it is convenient to describe stress at a pointwithin a component or a structure on an infinitesimal cube which centreson the point and whose faces are normal to the axes of a chosencoordinate system. The stress is resolved into three normal stresses andsix shear stresses acting on the faces of the cube. Since the choice ofthe coordinate system and its orientation is somewhat arbitrary or forthe convenience of analysis, the levels of the normal and shear stressesmay vary with the orientation of the coordinate system. There exists aspecial orientation of the coordinating system. On the faces of theinfinitesimal cube aligned to this particular coordinate system, thereare only resolved normal stresses and no resolved shear stresses. Thesespecial resolved normal stresses are called principal stresses (σ₁, σ₂,σ₃). The maximum principal stress (σ₁) is the greatest of the three andthe minimum principal stress (σ₃) the least.

von Mises Stress: The mechanical properties (modulus of elasticity,yield stress, work hardening and plastic deformation beyond yielding,etc.) of a ductile material such as an aluminium alloy are normallyestablished through tensile tests. Tensile tests are carried out underuniaxial stress conditions. Stress-strain curves are obtained. In orderto conduct stress analysis on a multi-axially stressed component orstructure, it is necessary to establish a correlation between themulti-axial stress-strain relationship and the uniaxial stress-strainrelationship, especially in the situation of material yielding whereHooke's law is no longer applicable. von Mises proposed a yieldcriterion which as been generally accepted as the most suitable for theductile materials.

Beyond yield, the von Mises stress and equivalent strain (defined in asimilar form to von Mises stress) will follow the tensile stress-straincurve. Therefore, von Mises stress may be generally used to assess theseverity of the stress state at any point of a component or structure,except when the component or structure is predominantly underhydrostatic tension. A gas cylinder is not under such stress condition.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is directed to the accompanying drawings in which:

FIGS. 1 and 2 are von Mises stress plots of parts of aluminium gascylinders, as discussed above.

FIGS. 3, 4 and 5 are bar charts of cycles to failure at test pressureshowing the effect of cylinder design and autofrettage on fatigue life.

FIGS. 6 and 7 are axial sections through pressure gas containers testedin Examples 4 and 5 below.

FIGS. 8a and 8 b show flat base and angled base designs referred to inExample 1.

DETAILED DISCUSSION OF PREFERRED EMBODIMENTS

The following Examples illustrate the invention. In all cases,autofrettage: was performed at ambient temperature; did move a region ofpeak stress at the closed end of the vessel to at least 0.5 mm away froman internal or external surface; and did not cause plastic deformationof the cylindrical side wall of the vessel.

EXAMPLE 1

A 7000 series alloy was used for this work, having the composition Zn5.96%; Mg 2.01%; Cu 1.87%; Cr 0.20%; Fe 0.06%; Si 0.03%; Balance Al.Billets were homogenised at 475 to 485° C., air cooled to ambienttemperature, and cold extruded in a backward direction. Necking wasperformed to form a high pressure gas cylinder, which was solution heattreated for 1 hour at 475° C. followed by 4.25 hours at 180° C.,resulting in a 0.2% proof stress value of about 450 MPa.

The number of loading cycles to failure at proof test pressure (34.5MPa) increased when a 85% autofrettage over-pressurisation was employed,with this increase consistently occurring when other options were usedto increase cycle life, e.g. increasing the internal knuckle radiusand/or introducing an angled rather than a square external base. Theresults are set out in Table 1 below. Besides increasing cycle life, theover-pressurisation was found to change the fracture mode, from baseseparation without over-pressurisation, to a leak from a radial crackwith the cylinder remaining in one piece after over-pressurisation.

TABLE 1 Over- pressurisation No. Fatigue Cycles Knuckle (% of burst tofailure Radius % pressure) Flat Base Angled Base Fracture Mode 13 04,396 4,600 Base Separation 13 85 4,725 5,249 Leak 15 85 5,439 6,503Leak

The flat base and angled base designs are shown in section in FIGS. 8aand 8 b respectively.

Further results of this work are shown in the accompanying FIGS. 3, 4and 5, each of which is a bar chart, with the length of the bar showingthe number of cycles to failure at test pressure.

The x axis information is listed below:

AMB—billet extruded at room temperature

135° C.—billet heat to 135° C. prior to extrusion

13—internal knuckle radius 13°

15—internal knuckle radius 15°

Angle—external base shape i.e. corner section

Flat—external base shape i.e. standard

The results of tests to date are:

FIG. 3: Effect of cylinder design and extrusion temperature on fatigueperformance at test pressure.

FIG. 4: As FIG. 3+effect of autofrettage at 70% of burst pressure.

FIG. 5: As FIG. 3+effect of autofrettage at 85% of burst pressure.

EXAMPLE 2

Autofrettage trials were performed on aluminium 6061 alloy cylinderswith the same dimensions as in Example 1. Test conditions were asfollows:

Service Pressure—12.4MPa

Test Pressure—20.7MPa

Autofrettage Pressure—27.6 MPa

Minimum burst pressure—31.0 MPa

Actual burst pressure—35.2-35.9 MPa

Fatigue test results are set out in the following Tables 2 and 3.

TABLE 2 Tested at “Test Pressure” Normal Cylinders AutofrettagedCylinders Cycles to Failure Cycles to Failure Serial No. (Leak) SerialNo. (Leak) 7 13,192 1 28,707 8 12,904 2 24,281 9 13,506 3 29,382 Average13,201 (1.00) Average 27,457 (2.08)

TABLE 3 Tested at “Service Pressure” Normal Cylinders AutofrettagedCylinders Cycles to Failure Cycles to Failure Serial No. (Leak) SerialNo. (Leak) 10 129,464 4 >300,000* 11 132,180 5 >500,000* 12 115,1506 >500,000* *Test terminated prior to failure.

EXAMPLE 3

Autofrettage trials were performed on 7XXX Series alloy cylindersfabricated using the route described in Example 1. In this example, thealloy composition used was Zn 5.91%; Mg 1.95%; Cu 2.03%; Cr 0.20%; Fe0.11%; Si 0.07%; Balance Al. The cylinder dimensions were: Externaldiameter 203 mm, wall thickness 10.7 mm, base thickness 16 mm and length1016 mm.

Four levels of autofrettage were used, namely 0, 75, 85 and 95% of theactual burst pressure (57.8±0.1 MPa). Fatigue test results obtainedusing test pressures of 31 and 37.2 MPa are set out in the followingTable 4.

TABLE 4 Cycles to Failure Maximum Test Autofrettage Pressure (% actualburst pressure) (MPa) 0 75 85 95 31.0 9,725 10,745 12,567 13,448 37.26,707  9,940

EXAMPLE 4

Autofrettage trials were performed on 7XXX series alloy cylindersfabricated using the route described in Example 1. The alloy compositionwas:

Zn 6.02, Mg 2.00, Cu 1.97, Cr 0.20, Fe 0.11, Si 0.06 (wt %) and balanceAl.

The 10 1 cylinder dimensions (FIG. 6) were as follows:

External diameter 176 mm Mean Wall thickness 8.9 mm Base thickness 12.5mm Overall length 600 mm

Four levels of autofrettage were used, namely, 0, 80, 85 and 90% of thetheoretical burst pressure (i.e. not the actual burst pressure as usedfor Examples 1-3).

Test conditions and cylinder specifications are listed below:

Service pressure 20 MPa Test pressure 30 MPa Minimum Theoretical Burstpressure 48.2 MPa Actual Burst pressure 51.0 MPa Autofrettage pressure0, 38.6, 41.0 and 43.4 MPa

Fatigue test results obtained at service pressure and at test pressureare outlined in Table 5.

TABLE 5 Maximum Cylinder Pressure During Fatigue OverpressurisationFatigue Life Test (MPa) (% of Minimum Burst (Cycles to Failure) 30  08338 30 80 10836 30 85 >12000 30 90 >12000 20  0 28,144 20 90 58,100

EXAMPLE 5

Autofrettage trials have been initiated on 6061 hoop wrapped gascylinders. The cylinder specifications (FIG. 7) were as follows:

External diameter 140 mm Mean wall thickness 5.9 mm Base thickness 8.1mm Overall length 465 mm

with a glass fibre composite wrap 1.15 mm thick applied to the barrelsection of the aluminium cylinder.

Three levels of autofrettage were used, namely, 118% of test pressure(standard treatment, 71% of minimum theoretical burst pressure), 80% ofminimum theoretical burst pressure, 90% of minimum theroetical burstpressure.

Test conditions and cylinder specifications are listed below:

Service Pressure 20 MPa Test Pressure 30 MPa Minimum Theoretical burstpressure 50 MPa

Fatigue test results are set out in Table 6.

TABLE 6 Maximum Cylinder Overpressurisation Fatigue Life Pressure DuringFatigue (% of minimum theoretical (Cycles to Test (MPa) burst pressure)Failure) 20 71 67,289 20 80 37,257 20 90 >78,000

EXAMPLE 6

Autofrettage trials were performed on 7xxx series alloy cylindersfabricated using the route described in Example 1.

The alloy composition was:

Zn 5.99% Mg 1.99% Cu 2.00% Cr 0.20%

Fe 0.071% Si 0.051% (wt %) and balance Al.

The cylinder dimensions were as follows:

External Diameter 176 mm Mean wall thickness 8.9 mm Base thickness 12.5mm Overall Length 600 mm Capacity 10 l

Autofrettage was carried out at 90% of the actual burst pressure.

Test conditions and cylinder specifications are listed below:

Service Pressure 20 MPa Test Pressure 30 MPa Minimum Theoretical Burst50.7 MPa Pressure Actual Burst Pressure 56.6 MPa Autofrettage Pressure 0and 50.9 MPa

Fatigue test results obtained at service pressure and at test pressureare outlined in Table 7 below.

TABLE 7 Autofrettage Fatigue Life Maximum Test (% of Actual Burst(Cycles to Pressure (MPa) Pressure) Failure) 20  0  40 036 20 95 210 000

EXAMPLE 7

The introduction of compressive stresses into the base region of acylinder by autofrettage are governed by the design of the cylinder baseand particularly the knuckle region. This is illustrated by reference tothree gas cylinders of different design, each 176 mm external diameterwith an operating pressure of 20 MPa, a wall thickness of 8.9 mm and aminimum base thickness of 12.5 mm. The external surface of the base ofeach vessel was effectively flat. The internal shape of the base of eachvesel was as follows:

a) This was an internal semi-ellipsoidal base design shown in FIG. 6.The internal surface of the base was concave with a depth (the dimensionQ of 30.5 mm.

b) This was a torispherical base design with a base depth of 36.85 mm.

c) This was another torispherical design with a base depth of 49.57 mm.

von Mises stress values in the knuckle regions of these gase cylindershave been calculated, and the results are set out in Table 8. Note thatthe autofrettage pressure used was 90% of the actual burst pressure.

The semi-ellipsoidal design a) introduces a peak stress at the internalsurface of the knuckle region of 306 MPa when subjected to its servicepressure of 20 MPa, and of 441 MPa when subjected to a test pressure of30 MPa. However, after autofrettage at 90% of the actual burst pressure,the stresses are reduced to 214 MPa and 288 MPa respectively within theknuckle region and 151 MPa and 242 MPa respectively at the internalsurface of the knuckle region.

Torispherical base designs generally exhibit lower stresses at operatingpressures. Thus torispherical base design b) introduces a peak stress

TABLE 8 von MISES STRESS VALUES IN THE KNUCKLE REGION OF 176 mm DIAMETERGAS CYLINDERS MAXIMUM von MISES STRESS (STRESS REDUCTION KNUCKLE INBRACKETS) RADIUS @ 20 MPa @ 30 MPa @ 20 MPa @ 30 MPa STRESS No No AfterAfter BASE DESIGN LOCATION Autofrettage Autofrettage AutofrettageAutofrettage SEMI-ELLIPSOIDAL Region 306.2 440.8 214.3 (91.9) 287.8(153)   Surface — —  151.2 (155.0) 241.7 (199.1) TORISPHERICAL 1 Region226.9 340.8 179.6 (47.3) 256.1 (84.2)  Surface — — 161.1 (65.8) 249.5(90.8)  TORISPHERICAL 2 Region 197   — 197 Surface — — 197

at the internal surface of the knuckle region, of 227 MPa at servicepressure, and of 341 at test pressure. However, autofrettage is stilleffective in reducing these stresses. After autofrettage at 90% of theactual burst pressure these stresses are 180 MPa and 256 MParespectively within the knuckle region and 161 MPa and 250 MParespectively at the surface of the knuckle region.

In both these cases a) and b) the stress levels at or adjacent theexternal surface at the centre of the base are reduced similarly, i.e.autofrettage introduces compressive stresses in this area also.

In designs a) and b) the region of peak stress after autofrettage waslocated in the knuckle more than 0.5 mm from the internal surface of thevessel.

Internal torispherical base design c) is more effective in reducingstresses at operating pressure than both the previous designs discussed.Thus at service pressure the highest stress predicted with FE analysiswas at the internal surface of the knuckle region and meansured 197 MPa.There was no reduction in this stress after autofrettage at 90% of theactual burst pressure. (A higher autofrettage pressure would have beeneffective to reduce the stress).

Although the torispherical base design c) does have advantages withrespect to standard operating conditions over the other two examples,i.e. lower stress, there are also several disadvantages:

i) Stresses cannot be reduced by autofrettage.

ii) Maximum stress is at the internal surface of the knuckle regionsurface, i.e. it cannot be moved internally to within the cylinder wall.

iii) Surface stress levels at the knuckle region are lower afterautofrettage for the ellipsoidal and torispherical designs a) and b).

iv) Without machining, the weight of a flat bottomed cylinder with atorispherical base design c) is greater than either of the two otherdesigns a) and b).

What is claimed is:
 1. A method of treating a pressure vessel comprisedof one of aluminium and an Al alloy, having a cylindrical side wall anda closed end and a service pressure and having, when at said servicepressure, at least one region of peak stress located at a surface, whichmethod comprises the steps of subjecting the inside of the vessel toautofrettage by applying a pressure sufficient to plastically deformsaid peak stress surface, said plastic deformation being confined toless than 25% of the wall thickness, whereby the treated pressure vesselhas the property that, when at said service pressure, each region ofpeak stress is located away from said peak stress surface at a distanceless than 25% of the wall thickness at said surface.
 2. A method asclaimed in claim 1, wherein the aluminium is a 7000 or 6000 or 2000Series alloy.
 3. A method as claimed in claim 1, wherein prior to saidsubjecting step, said method includes the step of forming the pressurevessel by backward extrusion.
 4. A method as claimed in claim 1, whereinplastic deformation of the metal takes place at an internal knucklewhere the closed end joins the side wall and/or axially of the vessel atthe outer surface of the closed end.
 5. A method as claimed in claim 1,wherein said applied pressure is such as not significantly toplastically deform the side wall of the vessel.
 6. A method according toclaim 1 wherein the pressure applied to achieve autofrettage is such asto limit plastic deformation of a side wall including said peak stresssurface to a thickness less than 10% of the side wall.
 7. A methodaccording to claim 1 wherein in the treated pressure vessel at elevatedpressure said region of peak stress is remote from any internal orexternal surface by at least 0.5 mm.
 8. A method according to claim 1wherein in the treated pressure vessel at elevated pressure said regionof peak stress is remote from any internal or external surface by adistance greater than a typical depth of surface flaws thereon.
 9. Apressure vessel of aluminium or an aluminium alloy having an axis, acylindrical side wall and a closed end joined to the side wall at aknuckle, and having the property that, when at elevated pressure, aregion of peak stress is located, within the material of the vessel awayfrom any internal or external surface at a distanceless than 25% of thewall thickness from said internal or external surface, in the knuckleand/or axially of the vessel in the closed end.
 10. A pressure vessel asclaimed in claim 9, wherein the aluminium is a 6000 or 7000 seriesalloy.
 11. A pressure vessel as claimed in claim 9, having the propertythat, when at elevated pressure, the local stress in the cylindricalside walls decreases from the inner surface to the outer surfacethereof.
 12. A pressure vessel according to claim 9 wherein at elevatedpressure said region of peak stress is remote from any internal orexternal surface by at last 0.5 mm.
 13. A pressure vessel according toclaim 9 wherein at elevated pressure said region of peak stress isremote from any internal or external surface by a distance greater thana typical depth of surface flaws thereon.